In one aspect, the present invention relates to poly(arylene sulfide) (PAS) resin compositions and to methods of producing PAS resin compositions. In another aspect, the present invention relates to continuous fiber-reinforced PAS composites and to methods of producing continuous fiber-reinforced PAS composites.
Due to their thermal resistance, chemical resistance, and desirable mechanical properties, PAS resins, e.g., poly(phenylene sulfide) (PPS) resins, are widely used for the production of coatings, fiber-reinforced composites, molded articles, laminates, and the like. Continuous fiber-reinforced PAS composites and laminates, for example, are lightweight, strong, non-rusting, chemical resistant, and heat resistant. Thus, PAS resin composites and laminates can advantageously be used to replace materials such as steel, wood, aluminum, etc. in the production of numerous items and structures (e.g., frames, supports, gratings, ladders, walkways, guardrails, tubes, pipes, etc.).
As is known in the art, the physical properties of a finished PAS resin product are determined, to a large extent, by the crystalline nature of the PAS resin. Solidification of the PAS resin to a fine-grained crystalline morphology improves the strength, impact resistance, heat deflection characteristics, and creep resistance of the PAS resin product and increases the product's maximum use temperature. When a PAS resin composition containing a reinforcing material is used, solidification of the PAS resin to a fine-grained crystalline morphology further improves the physical properties of the PAS resin product by improving the interfacial adhesion between the PAS resin and reinforcing material.
Various techniques have been used to improve the crystalline morphologies of finished PAS products. For example, the crystalline morphologies of some injection molded PAS products have been improved by using a relatively high temperature mold. The crystalline morphologies of certain compression molded PAS products have been improved through the use of a two-step procedure wherein the compression molded PAS product is rapidly cooled, i.e., quenched, to room temperature and subsequently annealed in order to produce a fine-grained crystalline structure.
As explained in U.S. Pat. No. 4,690,972, the entire disclosure of which is incorporated herein by reference, the crystalline morphology of a finished PAS product can also be improved through the incorporation of one or more nucleating agents. In the process of U.S. Pat. No. 4,690,972, a small amount of a nucleating agent (e.g., a poly(arylene ether ketone) or a poly(arylene sulfide/ketone)) is mixed with a PAS resin. The PAS/nucleating agent blend is then heated to a temperature above the melting point of the PAS resin and allowed to cool at a rate of less than about 50.degree. C. per minute. Although the manner in which the nucleating agent operates is not fully understood, it is believed that, by providing nucleating sites throughout the PAS resin, the nucleating agent facilitates the initiation of the crystallization process and causes rapid and extensive crystallization to occur in the PAS resin. The relative effectiveness of a given nucleating agent is measured by the amount by which the agent increases the melt crystallization temperature (T.sub.mc) of the PAS resin and/or the amount by which the agent reduces the size of the spheralitic structures which form in the resin during the crystallization process.
Using nucleating agents to obtain highly crystalline PAS products can provide significant advantages over the high temperature molding and quench/annealing techniques mentioned above. For example, using a nucleating agent to obtain a fine-grained crystalline morphology generally simplifies the crystallization process and reduces equipment requirements.
Unfortunately, current PAS/nucleating agent blending techniques of the type described in U.S. Pat. No. 4,690,972 do not provide PAS/nucleating agent blends which are well suited for use in the production of continuous fiber-reinforced composites. In U.S. Pat. No. 4,690,972, continuous fiber-reinforced PAS composites are prepared by: (a) grinding (e.g., air milling) a PAS resin and polymeric nucleating agent to a finely divided form; (b) blending the nucleating agent with the PAS resin before, during, or after the grinding procedure; (c) preferably, slurrying the finely divided PAS/nucleating agent mixture using an inert carrier liquid (e.g., water); and (d) pulling the continuous fiber-reinforcing material through the PAS/nucleating agent slurry. Due to the fact that the PAS resin and polymeric nucleating agent components of the PAS/nucleating agent blend generally have different susceptibilities to the grinding techniques used in the art, however, the PAS resin and nucleating agent components of the blend typically do not have the same particle size (i.e., the PAS resin particles are generally either larger or smaller than the nucleating agent particles). As is known in the art, the continuous reinforcement material will more readily retain the component of the PAS/nucleating blend which has the larger particle size. Consequently, blending and impregnation techniques such as those described in U.S. Pat. No. 4,690,972 do not provide uniform PAS resin and nucleating agent concentrations along the length and over the cross-section of the continuous reinforcement material.
Arylene sulfide polymers are typically produced by solution polymerization processes wherein at least one dihalo-arylene reactant compound is reacted with a sulfur source (e.g., an alkali metal sulfide) in the presence of a polar organic solvent. Modified arylene sulfide polymers can be obtained by adding polymerization modifying compounds to the reaction system. Examples of such polymerization modifying compounds include: polyhalo-arylene compounds, which can be added to the reaction system to promote polymer chain branching; monohalo-arylene sulfide compounds, which can be added to the reaction system to cause polymer chain termination; and alkali metal carboxylates, which can be added to the reaction system to promote the formation of a polymer of increased molecular weight. Upon the substantial completion of the polymerization process, the arylene sulfide polymer product is typically recovered from the reaction system using either a "solvent flashing" technique or a "quench recovery" technique.
The solvent flashing technique is described, for example, in U.S. Pat. Nos. 4,656,231 and 4,415,729, the entire disclosures of which are incorporated herein by reference. In the solvent flashing technique, the pressure of the reaction system is first reduced by an amount sufficient to cause a portion of the polar organic solvent to flash (i.e., vaporize) out of the reaction system. The resulting concentrated reaction system is then reheated and repressurized. Subsequently, the pressure of the concentrated reaction system is reduced by an amount sufficient to flash away the remainder of the polar organic solvent and leave a powdery, crude polymer product. The crude polymer product is then washed and dried in order to remove reaction system impurities.
The quench recovery technique is described, for example, in U.S. Pat. No. 4,415,729, the entire disclosure of which has been incorporated herein by reference. In the quench recovery technique, a molten polymer product phase is formed in the liquid reaction system. Subsequently, the temperature of the reaction system is reduced sufficiently (i.e., quenched) to cause the formation of a solid particulate polymer product. The solid polymer product is then filtered out of the reaction system, washed, and dried.
The PAS polymerization and recovery methods used heretofore have various shortcomings. The quench recovery and solvent flashing techniques discussed above yield crude polymer products which are loaded with impurities. The washing and drying procedures required to remove impurities from the arylene sulfide polymer product are time consuming and costly and typically result in significant polymer product losses. Additionally, the recovery processes described above typically yield feathery polymer products which have low bulk densities. Feathery polymer particles tend to plug the filtration equipment used to separate a quench recovered polymer from its reaction system while a low product polymer bulk density creates downstream processing and handling difficulties. The polymer product particles produced in the quench recovery process also tend to retain large amounts of the polymerization solvent. Solvent retention creates additional polymer purification and solvent recovery problems and leads to substantial product and solvent losses.
Therefore, a need presently exists for a PAS polymerization/recovery process which produces a compact, solid polymer product which is more readily separated from the polymerization system. A need also exists for a PAS polymerization/recovery process which produces a polymer product of increased bulk density. An additional need presently exists for a PAS polymerization/recovery process which yields a purer polymer product and reduces solvent retention. Further, a need currently exists for a PAS/nucleating agent mixing technique which provides a uniform PAS/nucleating agent blend which is well suited for use in the production of continuous fiber-reinforced composites.